Page 82 - Wastewater Solids Incineration Systems

Combustion Technology 51

(indirect-type fluid bed dryer by in-bed tube heat exchanger has been used largely in
the powder industries), and oil cracking.

2.4 Fluidization Gas Velocity
The term fluidization has been used in the literature to refer to dense-phase fluidiza-
tion and lean-phase fluidization and circulation systems involving pneumatic trans-
port or moving bed. In this manual the term is restricted to dense-phase or bubbling
fluid bed, which is mostly used for combustion of wastewater solids.
One of the most important factors in determining the regime of fluidization is the
gas velocity U . If the gas velocity is too low, then the bed becomes sluggish, which
0
could lead to severe consequences (such as agglomeration of sand in the vicinity of
the fuel guns or even explosion). If the gas velocity is too high, then fresh feeds could
bounce prematurely to the freeboard, leading to over-bed burning and lack of com-
bustion in the sand bed. This situation may cause higher fuel consumption and
incomplete combustion, leading to higher emissions. For a given bed of solids, the
fluidizing velocity of gas U is located between two limits: the minimum fluidization
0
gas velocity, U , and the terminal gas velocity, U .
mf t
2.4.1 Minimum Fluidization Gas Velocity U
mf
Estimation of the gas velocity U at the onset of fluidization is essential because it is
mf
the most fundamental design parameter in fluidization. It determines the transition
point between the fixed bed and the fluidized bed. Gas velocity U can be deter-
mf
mined by measuring the bed pressure drop as a function of gas velocity (Figure 5.3).
On the pressure drop curve versus gas velocity, U is the velocity at which a sudden
mf
reduction in pressure drop occurs.
The minimum fluidization gas velocity can also be estimated by the following
equation defined by Kunii and Levenspiel (1969) for small particles:
2
U d ( ) g/1650 (5.1)
mf p s g
Where
U gas velocity (cm/s),
mf
d particle diameter (cm),
p
3
density of solid (g/cm ),
s
3
density of gas (g/cm ),
g
2
g acceleration of gravity (980 cm/s ), and
viscosity of gas (g/cm s).
Per equation 5.1, U depends on characteristics of both solid particles and flu-
mf
idizing gas. It varies greatly with the particle diameter.